resolved: fix crash when shutting down

Reported by Cristian Rodríguez

http://lists.freedesktop.org/archives/systemd-devel/2015-May/031626.html

src/shared/prioq.c

@@ -45,12 +45,14 @@ Prioq *prioq_new(compare_func_t compare_func) {
         return q;
 }
 
-void prioq_free(Prioq *q) {
+Prioq* prioq_free(Prioq *q) {
         if (!q)
-                return;
+                return NULL;
 
         free(q->items);
         free(q);
+
+        return NULL;
 }
 
 int prioq_ensure_allocated(Prioq **q, compare_func_t compare_func) {

src/shared/prioq.h

@@ -28,7 +28,7 @@ typedef struct Prioq Prioq;
 #define PRIOQ_IDX_NULL ((unsigned) -1)
 
 Prioq *prioq_new(compare_func_t compare);
-void prioq_free(Prioq *q);
+Prioq *prioq_free(Prioq *q);
 int prioq_ensure_allocated(Prioq **q, compare_func_t compare_func);
 
 int prioq_put(Prioq *q, void *data, unsigned *idx);